Light olefins such as ethylene, propylene, etc are important chemical feedstocks, wherein propylene is a synthetic monomer of the products such as polypropylene, acrylonitrile, etc. Along with the rapid increase of the demand for the derivatives such as polypropylene, etc, the demand for propylene is also rapidly increasing year by year. The demand of the world market for propylene already increased from 15.2 million tons of 20 years ago to 51.2 million tons of 2000 with an average annual increase rate of 6.3%. It is predicted that the demand for propylene will have attained 86 million tons by 2010 with an average annual increase rate of 5.6%.
The process for producing propylene is mainly steam cracking and catalytic cracking (FCC), wherein steam cracking uses light oils such as naphtha, etc. as feedstocks to produce ethylene and propylene by thermal cracking with a propylene yield of only about 15% by weight, while FCC uses heavy oils such as vacuum gas oil (VGO) as a feedstock. Presently, 66% propylene in the world comes from the byproducts of steam cracking for producing ethylene, 32% comes from the byproducts of FCC of refineries for producing naphtha and diesel, and a small amount (about 2%) is obtained from the dehydrogenation of propane and metathesis of between ethylene and butene.
If the petrochemical industry produces ethylene and propylene through the conventional steam cracking route, several restrictive factors would be faced such as shortage of light feedstocks, deficiency of the processing capacity, and high production cost.
FCC is drawing increasing attention due to its advantages of wide adaptability, flexible operation, etc. In the United States of America, almost 50% of the demand of the market for propylene comes from FCC units. The development of the improved techniques of catalytic cracking for increasing the production of propylene is very rapid.
U.S. Pat. No. 4,980,053 discloses a conversion process for producing light olefins from hydrocarbons, wherein the feedstocks are petroleum fractions, residuum, or crude oil with different boiling ranges and the converting reaction is carried out in a fluidized bed or moving bed reactor under the conditions of a temperature between 500° C. to 650° C., a pressure between 1.5×105 and 3×105 Pa, a WHSV between 0.2 h−1 and 2.0 h−1, and a catalyst to oil ratio between 2 and 12 by using a solid acid catalyst. The catalyst returns to the reactor for cycle use after being regenerated by burning off coke. By this process, the total yield of propylene and ethylene may attain about 40%, wherein the yield of propylene is up to 26.34%.
WO 00/31215A1 discloses a catalytic cracking process for producing olefins, which uses a catalyst with ZSM-5 and/or ZSM-11 zeolites as active components and a great amount of inert substances as substrates, and uses VGO as a feedstock. The yield of propylene does not exceed 20% by weight.
U.S. Pat. No. 6,123,830 discloses a combination process consisting of two-stage catalytic cracking and two-stage hydrotreating, the objective of which is to produce as much olefins as possible and improve the quality of oil distillates and octane number of naphtha. The feedstock is converted into the first hydrotreated product in the first hydrotreating unit, and the first hydrotreated product enters the first catalytic cracking unit, wherein naphtha, diesel, and heavy oil are obtained by using a catalyst with an intermediate pore size zeolite as a main active component. The heavy oil enters the second hydrotreating unit for hydrogenation to obtain the second hydrotreated product, and the second hydrotreated product enters the second catalytic cracking unit for cracking, wherein the active component of the catalyst is mainly an intermediate pore size zeolite. The yield of propylene in this process is rather low.
Aromatics are also important chemical feedstocks, in particular, light aromatics such as BTX (benzene, toluene, and xylene), which are used to produce synthetic materials such as chemical fibers, plastics, etc. Presently, the major process for producing aromatics is catalytic reforming, wherein the feedstock should be fed to strict pretreatment because the active components of the reforming catalyst are noble metals. Disadvantageously the process flow of the movement and regeneration of the reforming catalyst is complicated.
The above references produce propylene only as a byproduct with low yields not exceeding 30% and at the same time that naphtha and diesel are produced. Several of the references only can produce aromatics, but cannot produce light olefins and aromatics simultaneously. In order to meet the increasing demand for the chemical feedstocks, propylene, ethylene, aromatics, etc, there is a necessity to develop a chemical industry type oil refining process for simultaneously producing large amounts of propylene, ethylene, and aromatics from heavy feedstocks.